Emulsion polymers used in paint formulations are generally prepared using a stabilizing agent or protective colloid in order to prevent coalescence of polymer particles, with subsequent precipitation of the particles or coagulation of the product. Examples of stabilizing agents include polyvinyl alcohol and water soluble cellulose derivatives such as carboxymethyl cellulose, hydroxy methyl, ethyl or propyl cellulose. Paints prepared with such emulsion polymers, however, are characterized by poor viscosity control and large particle size.
More recently, starch derivatives have been used as stabilizing agents in the preparation of emulsion polymers. U.S. Pat. No. 4,322,322 describes hydroxypropyl and hydroxyethyl starches which are used as protective colloids in the preparation of water containing vinyl acetate copolymer dispersions. U.S. Pat. No. 4,845,152 describes polyvinyl ester emulsions for use as tape joint compounds. The emulsions in U.S. Pat. No. 4,845,152 are prepared by the emulsion polymerization of vinyl ester monomers and polymerizable comonomers using as a stabilizer a hydrophobically modified starch, specifically an alkenyl succinate starch. However, the alkenyl succinate starch is prepared by a standard esterification process wherein the reagent and starch suspended in water are mixed under alkaline conditions. The disadvantage of using hydrophobically modified starch prepared by the standard process as a stabilizer for paint emulsions is large particle size and poor gloss.
U.S. Pat. No. 5,672,699 describes a method for preparing a hydrophobically modified starch wherein the starch and anhydride reagent are mixed at low acidic pH to form a stable dispersion before being brought to alkaline reaction conditions. The method allows for the use of reagents having long hydrophobic side chains, such as dodecenyl and hexadecenyl.
Thus, it would be advantageous to prepare a paint composition using an emulsion polymer which is prepared with a hydrophobically modified starch as a stabilizer which overcomes viscosity control problems, large particle size, and poor gloss which are characteristic of prior art paint compositions.